Comparison of the response to the prone position between pulmonary and extrapulmonary acute respiratory distress syndrome

Objectives:To determine whether the response to the prone position differs between acute respiratory distress syndrome (ARDS) resulting from a pulmonary cause (ARDS(p)) and that from an extrapulmonary cause (ARDS(exp)). Design and setting: Prospective observational study in a medical ICU of a university-affiliated hospital. Subjects: A consecutive series of 31 patients with ARDS(p) and 16 with ARDS(exp) within 3days of onset of ARDS. Intervention: Prone position for at least 2 h. Measurements and results: In ARDS(p), compared with the supine position (121 +/- 49 mmHg), PaO2/ FIO2 was not increased after 0.5 h but was increased after 2 h in the prone position (158 +/- 60 mmHg). In ARDS(exp), compared with the supine position (106 +/- 53 mmHg), PaO2/ FIO2 was increased after 0.5 h (155 +/- 91 mmHg), but was not further changed after 2 h. Marked oxygenation response (increase in PaO2/FIO2 > 40 % from baseline) after 0.5 h was 23 % in ARDS, and 63 % in ARDS(exp), and that after 2 h was 29 % and 63 %, respectively. Static respiratory compliance decreased in the prone position in ARDS(exp) (30 +/- 11 ml/cmH(2)O at baseline, 27 +/- 11 after 0.5 h and 25 +/- 9 after 2 h) but not in ARDS(p). Consolidation score as determined on the first chest radiography taken in the prone position decreased to a greater degree in ARDS (-2.4 +/- 4.1) than in ARDS(p) (0.3 +/- 4.1). Conclusion: Pulmonary ARDS and extrapulmonary ARDS in their early stages respond differently to the prone position with regard to the time course of oxygenation, respiratory mechanical behaviour, and radiographic change. These findings suggest that the early pathophysiology of ARDS differs according to the type of primary insult to the lung.